This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-218316, filed Aug. 2, 1999; No. 2000-107468, filed Apr. 10, 2000; No. 2000-122157, filed Apr. 24, 2000; and No. 2000-163303, filed May 31, 2000, the entire contents of which are incorporated herein by reference.
This invention relates to a photosensor for sensing light or an image according to the sensed light and a photosensor system.
One known two-dimensional image reading device for reading printed matter, photographs, or fingerprints by very small irregularities in the finger has a photosensor array composed of photoelectric conversion elements (photosensors) arranged in a matrix. Generally, a solid-state imaging device, such as CCD (Charge-Coupled Device), made of single crystal silicon has been used as a photosensor array. Use of single crystal silicon causes the problem of increasing the manufacturing cost seriously.
It is well known that the CCD has a structure where photodiodes or photosensors are arranged in a matrix, causes a horizontal scanning circuit and a vertical scanning circuit to detect the charges generated according to the amount of light projected on the light-receiving section of each photosensor, and senses the luminance of the projected light. In a photosensor system using such a CCD, because select transistors for respectively bringing the scanned photosensors into the selected state have to be provided independently, an increase in the number of pixels causes the problem of making the overall system larger.
To overcome this problem, an attempt has been recently made to make the system smaller and reduce the manufacturing cost by applying a thin-film transistor with a so-called double-gate structure (hereinafter, referred to as a double-gate photosensor) to an image reading device. The double-gate photosensor is such that a photosensor has a photo sense function and a select transistor function.
The plane structure of a photosensor array composed of such double-gate photosensors PS is so designed, for example, as shown in FIG. 30 that double-gate photosensors PS are arranged with a specific pitch of Psp in a lattice-like form (or in a matrix) in the directions of x and y crossing at right angles and that light from the insulating substrate (or glass substrate) side is projected through the element-to-element region Rp in the lattice onto the subject. Therefore, to project sufficient light on the subject to improve the light-receiving sensitivity, it is necessary to make the element-to-element region Rp as large as possible.
FIG. 31 is a sectional view showing the structure of a double-gate photosensor PS taken along line XXXIxe2x80x94XXXI in FIG. 30. The double-gate photosensor PS comprises a semiconductor layer 1 where electron-hole pairs are generated by incident light, n+ silicon layers 7 provided at both ends of the semiconductor layer 1, a source electrode 2 and a drain electrode 3 which are formed on the n+ silicon layers 7 and shut off light exciting the semiconductor layer 1, a block insulating film 4 provided on the semiconductor layer 1, an upper gate insulating film 5 covering the source electrode 2 and drain electrode 3, a top gate electrode TG formed on the upper gate insulating film 5, a lower gate insulating film 6 below the semiconductor layer 1, a bottom gate electrode BG which is formed below the lower gate insulating film 6 and shuts off light exciting the semiconductor layer, and a transparent substrate 9.
Specifically, the double-gate photosensor PS is such that a combination of two MOS transistors, of an upper MOS transistor composed of the semiconductor layer 1, source electrode 2, drain electrode 3, and top gate electrode TG; and a lower MOS transistor composed of the semiconductor layer 1, source electrode 2, and drain electrode 3, and bottom gate electrode BG, is formed on the transparent insulating substrate 9, such as a glass substrate using the semiconductor layer as a common channel region.
Then, light hxcexd emitted from above the double-gate photosensor PS advances in the direction of the arrow, passes through the top gate electrode TG and transparent insulating films 4, 5, and enters the semiconductor layer 1. In the semiconductor layer 1, electron-hole pairs are generated according to the amount of incident light. By sensing the voltage signal corresponding to the charges, the light-and-shade information on the subject is read.
A photosensor system applied to the aforementioned two-dimensional image reading device has the following problems.
(a) The semiconductor layer 1 in a double-gate photosensor PS is set on the basis of various dimensions determining the channel region, that is, of the ratio of the channel length L0 to channel width W0 in the semiconductor layer 1. The channel length L0 coincides with the length of the block insulating film in the direction of channel length.
The transistor characteristic of the double-gate photosensor PS is generally expressed by the following expression (1):
xe2x80x83Idsxe2x88x9dW0/L0xe2x80x83xe2x80x83(1)
where Ids is a source-drain current value.
The double-gate photosensor system recognizes an image by reading the voltage at the drain electrode 3 that varies with the drain current Ids flowing on the basis of the charges generated in the semiconductor layer 1 according to the amount of incident light. Therefore, to clearly recognize the image of the subject in a high contrast ratio, the difference between the drain current Ids of a double-gate photosensor PS positioned in a dark portion of the subject and the drain current Ids of a double-gate photosensor PS positioned in a bright portion of the subject has to be made larger. Since the source-drain current value Ids that determines the transistor sensitivity of the double-gate photosensor PS is determined on the basis of the ratio of the channel width W0 to channel length L0 in the semiconductor layer 1, it is desirable from the viewpoint of improvement in the transistor sensitivity of the double-gate photosensor PS that the design value of the ratio W0/L0 should be made as large as possible.
On the other hand, if the ratio W0/L0 it set to be so that the double-gate photosensor PS is set to a high transistor sensitivity, the plane structure of the semiconductor layer 1 inevitably takes the form of a rectangular shape with a relatively large channel width W0 and a relatively small channel length L0. Because the double-gate photosensor PS senses only the light caused to enter the semiconductor layer 1, only the part not covered by the shade source electrode 2 and drain electrode 3 senses the light entering from above. As shown in FIG. 30, the area in which the light from the semiconductor layer 1 is allowed to enter takes a form of a near rectangle Ip0 with the length of the shorter side being K0 and the length of the longer side being about W0. Since the short-side length K0 basically depends largely on the channel length L0, when the light entering the semiconductor layer 1 is perfect diffuse light or almost perfect diffuse light, the amount of light entering the semiconductor layer 1 in the direction of x is smaller than the amount of light entering the semiconductor layer 1 in the direction of Y, resulting in a noticeable deviation of the incident light in the direction in which it travels.
Specifically, in such a double-gate photosensor PS, because the area of the semiconductor layer 1 constituting the channel region which light is allowed to enter is designed to take the form of a single rectangle Ip0, the light transmitting area at the surface of a protective insulating film that a single double-gate photosensor PS can basically sense is a lengthwise area Ep0 (the area shaded with slanted lines in FIG. 30) substantially similar in shape to a near rectangle Ip0, which narrows, in the sidewise direction (or the direction of x), the area assuring the desired light-receiving sensitivity. Thus, the deviation of the expanse of the light sensing area in the directions of x and y causes distortions in the image read, which prevents the light-and-shade information on the subject from being read accurately. This causes the problem of being unable to simultaneously realize both a high transistor sensitivity and the reading of image information with suppressed distortion. The area Ep0 does not represent the distribution range of the light-receiving sensitivity of the double-gate photosensor PS accurately.
(b) When double-gate photosensors PS are arranged in a matrix, the distances between the light-receiving sections are non-uniform in an oblique direction (0 to 90xc2x0) other than the two perpendicular directions (or the directions of x and y) corresponding to the matrix, leading to the deterioration of the reading accuracy. Specifically, the arrangement of double-gate photosensors PS in a photosensor array has the following problem: because double-gate photosensors PS are arranged in only two directions of x and y perpendicular to each other in such a manner that they are spaced at regular intervals of dimension Psp, the pitch between the double-gate photosensors PS increases in the directions of x and y non-uniformly (for example, {square root over ( )}2 times for 45xc2x0) in an oblique direction (at a suitable angle other than 0xc2x0, 90xc2x0, 180xc2x0, and 270xc2x0, for example, in the direction of 45xc2x0 or 60xc2x0) with respect to the directions of x and y corresponding to the matrix, which prevents the object put obliquely from being read uniformly with high accuracy.
(c) The difference in the reflection of the projected light due to regularities in the finger is sensed using carriers generated in the semiconductor layer 1 composed of axe2x80x94Si excited when light of visual light region is incident. Since the top gate electrode TG for accumulating carriers intervenes between such a subject as a finger and the semiconductor layer 1, it has the property of reflecting the light from the subject and allowing light in the wavelength region exciting the semiconductor layer 1 to pass through. For this reason, such a transparent electrode as ITO (Indium-Tin-Oxide) has to be used. The top gate electrodes TG of the double-gate photosensors PS adjacent to each other in the direction of row (or the direction of x) are connected to each other via top gate lines TGL. The top gate lines TGL themselves are made of ITO in such a manner that they are formed integrally with the top gate electrodes TG. ITO has a higher resistivity than such metallic material as chromium widely used as a wiring layer, which causes the problem of tending to cause a delay in the propagation of signals.
To solve the high-resistance ITO problem, the wiring resistance is reduced by forming a top gate line TGL composed of a wider wiring layer and making the wiring cross-sectional area larger. Even a transparent electrode like ITO attenuates the amount of transmitting light, which causes a problem: when the electrode is made thicker carelessly, the light-receiving sensitivity decreases.
In the manufacturing processes of the individual component parts of double-gate photosensors, top gate lines TGL are formed at a relatively upper layer, after various wiring layers, including drain lines DL connecting the drain electrodes 3 of the double-gate photosensors PS arranged adjacent to each other in the direction of column (or the direction of y), source lines SL (or ground lines) connecting the source electrodes 2 of the double-gate photosensors PS arranged adjacent to each other in the direction of column, and bottom gate lines BGL connecting the bottom gate electrodes BG of the double-gate photosensors PS arranged adjacent to each other in the direction of row, have been formed in a stacked manner. Therefore, the top gate lines TGL are liable to be influenced by steps in the stacked structure, which causes a problem: there is a strong possibility that the wires will break.
Furthermore, since the top gate lines TGL overlap with the bottom gate lines BGL between the double-gate photosensors PS adjacent to each other in the direction of row, the overlap capacitance between the top gate lines TGL and the bottom gate lines BGL introduces the problem of being more liable to cause a delay.
Accordingly, a first object of the present invention is to provide a photosensor system which improves the deviation of the light sensing area and has high-transistor-sensitivity photosensors and photosensors arranged therein to achieve a well-balanced light-receiving range.
A second object of the present invention is to provide a photosensor system which solves the aforementioned problems and can be driven properly, while suppressing a delay in the signal.
To achieve the first object, a photosensor according to the present invention includes one or more semiconductor layers having carrier generating regions for generating carriers when being struck by exciting light or channel regions in which drain current flows. As a result, the positions of the carrier generating regions can be set arbitrarily in such a manner that the balance of incident light in the direction of two-dimensional travel is equalized. This enables sensing with less distortion and provides a large drain current, achieving a good transistor sensitivity. When carrier generating regions are provided in the direction of the channel length of the drain region, this brings particularly good results.
Furthermore, a delta arrangement of such photosensors equalizes further the distance between photosensors adjacent to each other two-dimensionally. This suppresses a shift in the optical information due to non-uniformity of the light-receiving sensitivity differing with the direction, when the same subject is placed on the photosensor array at a different angle in a plane. As a result, there are fewer limits to the angle at which the subject is placed, which helps realize a photosensor system with a far better image reading characteristic.
To accomplish the second object, a photosensor system includes a first gate line connecting the first gate electrodes of photosensors to each other and a second gate line connecting the second gate electrodes of the photosensors and arranged apart from the first gate line in a plane between the adjacent photosensors, thereby eliminating a parasitic capacitance between the first gate line and second gate line, which suppresses noise in the signal outputted to the first and second gate lines. Therefore, even if the number of photosensors connected to the first and second gate lines is very large, the system can operate properly. In addition, the voltage of the signal can be made relatively low, which reduces the power consumption. Moreover, branching the second gate line into plural lines produces the following effect: when the second late lines are formed at relatively higher layers in the stacked structure of the photosensor array, even if one second gate line has been broken due to steps, the signal can be supplied because the other second gate line compensates for the braking. This not only improves the yield in manufacture but also increases the cross-sectional area of the gate line wires to decrease the wiring resistance, thereby suppressing a delay in the propagation of the signal, which achieves a good subject image reading operation.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.